Relief mechanism for jaw crusher

ABSTRACT

A relief mechanism for a jaw crusher which includes a hydraulic fluid filled piston-cylinder arrangement mounted on the frame and connected to the rear toggle of the jaw crusher. The hydraulic fluid is connected to a pressure sensor and a relief valve. The hydraulic fluid is maintained at a predetermined pressure during normal crushing operations. Upon encountering a material which cannot be crushed, the rear toggle transmits the higher than set pressure to the piston-cylinder arrangement thereby increasing the fluid pressure in the piston-cylinder arrangement. A pressure sensor senses this increased pressure and opens the relief valve to dump the entire hydraulic system. As the hydraulic system is dumped, the piston moves into the cylinder. The relief valve is sized so that the fluid is exhausted from the cylinder at a rate which allows unrestrained movement of the piston during relief. A positive stop on the piston-cylinder arrangement insures pressurizing of the system. An interlock with the crusher motor may be provided to shut down the crusher when tramp is encountered.

[ 51 Nov. 11, 1975 RELIEF MECHANISM FOR JAW CRUSHER ABSTRACT Inventor: James Anthony, Allentown, Pa.

[73] Assignee: Fuller Company, Catasauqua, Pa. A relief mechanism for a jaw crusher which includes a May 31, 1974 Appl. No.: 475,002

hydraulic fluid filled piston-cylinder arrangement [22] Filed:

mounted on the frame and connected to the rear toggle of the jaw crusher. The hydraulic fluid is connected to a pressure sensor and a relief valve. The hydraulic fluid is maintained at a predetermined pressure during normal crushing operations. Upon encountering a material which cannot be crushed, the rear toggle transmits the higher than set pressure to the pis ton-cylinder arrangement thereby increasing the fluid pressure in the piston-cylinder arrangement. A pres References Cited UNITED STATES PATENTS sure sensor senses this increased pressure and opens the relief valve to dump the entire hydraulic system.

l.603.765 Haas et al.

As the hydraulic system is dumped, the piston moves 3.099.406 7/1963 Kautz........... 3.122992 3/1964 into the cylinder. The relief valve is sized so that the fluid is exhausted from the cylinder at a rate which al- FOREIGN PATENTS OR APPLICATIONS lows unrestrained movement of the piston during relief. A positive stop on the piston-cylinder arrangement insures pressurizing of the system. An interlock with the crusher motor may be provided to shut down the crusher when tramp is encountered.

V. n a m r e G 1.276.422 Germany Primary E.\'aminerGranville Y. Custer, Jr. Assistant E.\'aml'nerI-Ioward N. Goldberg Attorney, Agent. or Firm-Frank H. Thomson 6 Claims, 4 Drawing Figures US. Patent N0v.11 1975 Sheet10f2 $918,648

US. Patent Nov. 11, 1975 Sheet2 of2 3,918,648

FIG. 3

RELIEF MECHANISM FOR .IAW CRUSHER BACKGROUND OF THE INVENTION This invention relates to jaw crushers and in particular to a fluid relief mechanism for jaw crushers to prevent damage to the crusher in the event tramp is encountered.

It is known that a rock crusher should include some means for relieving the crusher in the event an uncrushable material such as tramp iron enters the crushing chamber. One relief mechanism which has been employed in jaw crushers is shown in U.S. Pat. No. 2,767,597. In this arrangement, the relief mechanism takes the form of a fracturable rear toggle. Although this arrangement performs well in relieving a jaw crusher, it has the disadvantage that in the event of a failure due to an uncrushable material, the toggle fractures and the crusher must be shut down and the toggle repaired. This is a time consuming operation and re- A sults in a great deal of crusher down time.

A mechanical relief mechanism such as that of U.S. Pat. No. 2,767,597 has additional disadvantages. The fracturable toggle is continuously subjected to cyclical loads and failure can occur due to fatigue. Because failure can occur due to fatigue, a mechanical toggle is not as reliable as is desirable. By its very nature, a mechanical relief system cannot be designed so that it will always fail at the designed load. One unit may fail at a different force than other units, and it may fail at a greater or lesser force than the designed force. Thus, the systems reliability is less than desired.

An additional disadvantage of a mechanical relief mechanism is that it is not adjustable. Once the force at which relief will occur is selected for a particular system, it cannot be changed without changing the fracturable toggle.

It would be desirable to provide some form of a relief mechanism which will relieve a jaw crusher by opening up the swing jaw in the event of an uncrushable material, but which will not result in lengthy down time after relief. It would also be desirable to provide a relief mechanism which is reliable, adjustable and repeatable. It is believed that some form of hydraulic relief will accomplish the desired objectives.

Prior to the present invention, hydraulic relief mechanisms have been successfully applied to gyratory crushers. Attempts have been made to apply hydraulic relief mechanisms to jaw crushers such as shown in U.S. Pat. No. 2,982,481. As applied to jaw crushers, such relief mechanisms serve only to absorb shock and the crusher continues to operate. If the tramp does not pass through the crusher when the swing jaw is opened during the shock absorption, when the shock absorber returns to its normal position, the swing jaw will again move toward the fixed jaw and again strike the tramp. Each time the swing jaw strikes the uncrushable material, damage can be done to the crusher.

Prior to the present invention those hydraulic relief mechanisms which have been successfully applied to gyratory crushers have not been applied to jaw crushers. In a gyratory crusher, the weight of the gyrating head assists in the operation of the relief mechanism. In addition, the weight of the gyratory member is such that the pressure required to hold the head in position permits the hydraulic system to be properly pressur ized. Hydraulic relief mechanisms are shown in U.S. Pat. Nos. 2,667,309 and 3,133,706.

By the present invention, l have provided an arrangement which will relieve the crusher when an uncrushable material such as tramp iron is encountered. The hydraulic system will be emptied at a rate at least equal to that required to permit free movement of the relief piston at the crusher operating speed. The swing jaw moves far enough away from the fixed jaw to clear most tramp. Even if tramp does not clear itself, the movement of the swing jaw will reduce the load on the machine. Once the crusher has relieved, the crusher can be shut down, the tramp removed from the crushing chamber, the system repressurized and the crusher restarted. The crusher downtime will be reduced to slightly more than the time necessary to remove the tramp. With a hydraulic relief mechanism, the force at which the crusher can be relieved can be adjusted without changing the system. The system is reliable because fatigue loads are not a factor. The system is repeatable because the same pressure can be reset after a relief has occurred.

SUMMARY It is therefore the principal object of this invention to provide a relief mechanism for a rock crusher which will reduce downtime in the event the relief mechanism is actuated.

It is an additional object of this invention to provide a relief mechanism for a jaw crusher which will substantially reduce load on the crusher mechanism in the event an uncrushable material is encountered.

It is a further object of this invention to provide a relief mechanism for a jaw crusher which is adjustable and insures that the same relief point can be reset after relief has occurred.

In general the foregoing and other objects of this invention will be carried out by providing for use in a jaw crusher including a frame, a fixed jaw mounted on said frame and a movable jaw mounted on said frame and means for moving said movable jaw alternately toward and away from said fixed jaw at a predetermined operating speed to thereby crush material between said fixed and movable jaws, apparatus for relieving excessive loads applied to said movable jaw, comprising piston-cylinder means adapted. to contain a volume of fluid and including a piston means mounted in a cylinder means; one of said piston means and said cylinder means operatively connected to the movable jaw of the jaw crusher and movable relative to the other of said piston means and cylinder means and said other of said piston means and cylinder means being fixed to said frame; means operatively connected to said pistoncylinder means for exhausting fluid in said pistoncylinder means when the fluid pressure in said pistoncylinder means exceeds a predetermined maximum whereby when an excessive load is applied to the movable jaw, it is transferred to fluid through the said one of the said piston means and cylinder means which is operatively connected to the movable jaw; said means for exhausting fluid including valve means for exhausting fluid from the piston-cylinder means at a flow rate which is at least equal to that required to permit said one of said piston means and cylinder means to move freely relative to said other of said piston means and cylinder means at the predetermined operating speed of the movable jaw when an excessive load is applied to the movable jaw.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in connection with the annexed drawings herein:

FIG. 1 is a sectional view of a typical double-toggle jaw crusher employing the present invention;

FIG. 2 is a fragmentary view on an enlarged scale of a portion of the relief mechanism of the present invention',

FIG. 3 is a diagrammatic view of the hydraulic circuit of the present invention; and

FIG. 4 is a sectional view of the jaw crusher of FIG. 1 after relief has occurred.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, there is shown in FIG. 1 a typical double-toggle jaw crusher which may employ the present invention. The crusher is generally indicated at 1 and includes a frame 2 having a fixed jaw 3 mounted thereon. The fixed jaw is conventionally provided with a replacable wearable material 4.

A swing jaw generally indicated at 6 is pivotally mounted on the frame 2 by means of a pivot shaft 7. The swing jaw 6 may also include a wearable material surface 8. In order to move the swing jaw 6 there is provided a means 9 for moving the swing jaw towards and away from the fixed jaw 3. This means 9 includes a shaft 10 rotatably mounted on the frame 2 and operatively connected to a meanssuch as an electric motor (52 in FIG. 3) for a rotating the shaft at a predetermined operating speed. A fly wheel 11 is also suitably connected to the shaft 10.

A Pitman 12 is eccentrically mounted on the shaft 10 in a well-known manner. The Pitman has a forward toggle 15 and a rear toggle 16 operatively connected thereto with the forward toggle 15 extending between the Pitman 12 and the rear of the swing jaw 6. The rear toggle 16 extends between the Pitman 12 and the rear of the frame 1.

In the present invention the rear toggle 16 extends between the Pitman 12 and apparatus for relieving excessive loads applied to the movable jaw 6, generally indicated at 20. During normal operation, the relief mechanism is fixed to the frame 1 so that the rear toggle 16 acts as a fixed point. In a well known manner, as the shaft 10 is rotated the Pitman 12 is reciprocated in a generally vertical direction. This vertical movement produces a generally horizontal movement of the forward toggle l5 and a resulting swinging movement of the swing jaw 6. As rock is fed into the feed opening 17 of the crusher and falls through the crushing chamber, it is crushed as the swing jaw 6 moves toward the fixed jaw 3. When the swing jaw moves away from the fixed jaw, the crushed material is dropped out of the bottom of the crushing chamber.

The relief mechanism of the present invention is best shown in FIGS. 2 and 3..The relief mechanism 20 includes a piston-cylinder means 21 havingcylinder means 22 secured to the frame 1 and a piston means 23 slidably mounted in the cylinder 22. An end plate 24 closes the end of the cylinder 22. A stop 25 is provided on the cylinder 22 and piston 23 to limit the forward movement of the piston 23 relative to the cylinder 22 and frame 1. The cavity 26 in the piston-cylinder means 21 is filled with a suitable liquid.

The forward face of the piston 23 may be provided with a bearing surface 27 and the toggle 16 with a hearing member 28 to thereby provide a pivotal connection between the toggle 16 and piston 23.

Referring to FIG. 3, there is shown a sump 30 for hydraulic fluid with a pump 31 for conveying the fluid in the sump 30 through a line 32 and check valve 33 and line 34 to the cavity 26 of the piston-cylinder means 21. As hydraulic fluid fills the cavity 26 and fluid is continued to be supplied to the piston-cylinder means 21, the fluid serves to advance the piston 23 forward to the stop 25 and pressurize the fluid in the piston-cylinder means 21 to a fixed, predetermined pressure. This pres sure will correspond with the maximum pressure at which the operator desires to crush rock. Any uncrushable material, such as tramp iron, will produce an excessive pressure on the swing jaw 6 which will be transferred through toggles 15 and 16 to the piston 23 and hence to the hydraulic fluid in cavity 26.

A pressure sensor 40 is provided in the line 34 connecting the pump 31 and the piston-cylinder means 21. This pressure sensor 40 is operatively connected to a solenoid valve 41, either mechanically or electrically, which is in turn operatively connected to a relief valve 42. The relief valve 42 is flow connected to the line 34 by a line 43 and to the sump 30. If the relief valve 42 is open, fluid from line 34 is dumped from the cavity 26 into the sump 30.

During operation of the crusher, so long as the material in the crushing chamber is capable of being crushed by a force within the pressure setting of the machine, the pressure in the piston-cylinder means 21 will be sufficient to hold the rear toggle 16, and all movement of the Pitman will be transferred to the forward toggle 15 and swing jaw 6. In the event some material enters the crushing chamber which requires a crushing force which exceeds the pressure setting of the machine, this force will be transmitted from the movable jaw 6, through toggles 15 and 16 to piston-cylinder means 21 increasing the pressure in piston-cylinder means 21 to a level above the predetermined maximum or setting. This increase in pressure will be sensed by pressure sensor 40. When the pressure sensor senses a pressure which exceeds the predetermined maximum, it will operate solenoid valve 41 which in turn operates relief valve 42 to open communication between cavity 26 and sump 30 through lines 34 and 43. With this communication established, the piston means 23 will move freely into the cylinder 22 so that the motion of Pitman 12 will now be translated into movement of the piston 23 and not movement of the swing jaw 6.

The relief valve 42 and lines 34 and 43 are sized to insure that all fluid in the cavity 26 is emptied from the cavity 26 at a rate at least equal to that required to permit the piston means 23 to move freely into the cylinder means 22 at the normal operating speed of the crusher. This insures that sufficient fluid in the pistoncylinder means 21 is exhausted from the pistoncylinder means to permit movement of the piston with substantially no resistance from the fluid. The weight of the swing jaw 6 and the resistance of the material in the crushing chamber is such that the piston 23 will move rearwardly in the cylinder 22 until the piston contacts the end plate 24 as shown in FIG. 4. Because of the rate at which the fluid is exhausted from the piston-cylinder means 21, substantially no force is imparted to the swing jaw 6 until the piston means 23 contacts the end plate 24. Once this contact occurs. force will again be imparted to the swing jaw 6, but at this point the crusher has opened up. As shown in FIG. 4, the geometry of the rear toggle 16 and Pitman I2 is such that the crusher opens more than the length of travel of the piston 23. The amount the crusher opens can best be appreciated by comparing the distance A in FIG. 1 which is the normal opening of the machine with the distance B in FIG. 4 after relief. The latter distance is sufficient to permit most tramp to pass through the machine after relief. With prior relief mechanisms, the amount the swing jaw opens is generally determined by the amount the relief mechanism moves. However, with the present relief mechanism, the swing jaw movement is substantially greater. Although movement imparted to the swing jaw 6 is the same as during normal operation, because the swing jaw has moved away from the fixed jaw, the swing jaw will not move as close to the fixed jaw after relief. Thus, even if the tramp remains in the crusher, the damaging force applied to the crusher by continued operation of the swing jaw against the un crushable will be substantially reduced.

In order to achieve the desired result of substantially no force being applied to the swing jaw during initial rearward movement of the piston means 23, it is important to properly size the relief valve and the lines 34 and 43 so that when an excessive load is applied to the swing jaw at the operating speed of the crusher the fluid in piston-cylinder means 21 is exhausted therefrom at a rate at least equal what the volume change in the piston-cylinder means 21 and cavity 26 would be if no fluid was in the cavity 26. This is doneby determining the amount of fluid which must be exhausted and the rate at which it must be exhausted. The rate at which the fluid must be exhausted depends on the rate of movement of the piston 23 into cylinder which'is directly related to the speed of operation of the crusher as determined by the speed of rotation of shaft 10.

The amount offluid which must be exhausted is determined by the size of the piston-cylinder means 21. The piston-cylinder size is calculated by using the force required to crush rock and the pressures to which the machine is designed. The stroke length of the swing jaw is also a factor in determining the volume of fluid .to be dumped. t

Although only a single relief valve 42 has been shown, it should be obvious that more than one such valvemay be used and will probably be required. A single pressure sensor 40 maybe used to operate a plurality of solenoid valves 41.

After the crusher has been relieved, the operator will shut down the crusher and remove material in the crushing chamber including the tramp. The pistoncylinder means is again pressurized and the crusher started. The down time of the machine after release is short once the tramp has been removed because no crusher parts need to be replaced.

In order to provide an automated system, the relief valve 40 is operatively connected to an alarm and interlock circuit 50 which is in turn operatively connected to an alarm 51 and the motor 52 for. rotating the shaft of the crusher. In the event the fluid pressure in the cavity 26 exceeds the predetermined pressure which activates the valve means 42 to dump the hydraulic system, the pressure sensor 40 also sends a signal to the alarm and interlock circuit 50 which shuts off motor 52 to stop the crusher and activates alarm 51 to tell the op erator of the relief.

The system may also include a second pressure sensor 54 to sense when the pressure is below a predetermined minimum. This sensor is also operatively connected to the motor means 52 through interlock and alarm circuit 50 so that the motor52 and crusher cannotbe operated until the relief mechanism has been set by pressurizing piston-cylinder means 21. In theory, the minimum and maximum pressure settings are the same, but in practice they should be different to provide for smooth operation.

" If desired, the high and low pressure sensors can also be tied into the feeder which supplies material to the crusher. All of this insures that if there is a line blockage or leakage, the potential for damage to the crusher is substantially reduced and that the amount of unc'rushed rock which passes through the machine is kept to a minimum.

If desired, an accumulator 35 may be placed in the line 34 for the purpose of absorbing minor shocks typical of a hydraulic system. This accumulator is not designed to absorb any relief in the event an uncrushable material is encountered. In the event of an uncrushable material the accumulator 35 will be exhausted through the lines 34 and 43 and valve 42 to the sump 30 along with the piston-cylinder means 21.

From the foregoing it should be apparent that the objects of this invention have been carried out. A relief mechanism has been provided for a jaw crusher which ensures relief in the event of an uncrushable material and can be easily-reset to the same pressure setting after the uncrushable material is encountered. Thus the system is repeatablepThe system is adjustable simply by changing the setting of the pressure sensors. By its very nature the relief mechanism is not subject to failure due to fatigue loads. Thus the machine is reliable.

It is intended that the foregoing description be merely that of apreferred embodiment and that the invention be limited solely by that which is within the scope of the appended claims.

I claim:

1. For use in a jaw crusher including a frame, a fixed jaw mounted on said frame and a movable jaw mounted on said frame and means for moving said movable jaw alternately toward and away from said fixed jaw at a predetermined operating speed to thereby crush material between said fixed and movable jaws, apparatus for relieving excessive loads applied to said movable jaw comprising: i piston-cylinder means adapted to contain a volume of fluid and including a piston means mounted in a cylinder means; I

said piston means operatively connected to the movable jaw of the jaw crusher and movable relative to the cylinder means and said cylinder means being fixed to said frame;

' means operatively connected to said piston-cylinder means for exhausting fluid in said piston-cylinder means when the fluid pressure in said pistoncylinder means exceeds a predetermined maximum whereby when an excessive load is applied to the movable jaw, it is transferred to the fluid through "the piston means; i

said means for exhausting fluid including valve means for exhausting fluid from the piston-cylinder means at a flow rate which is at least equal to that required to permit said piston means to move freely relative to said cylinder means at the predetermined operating speed of the movable jaw when an excessive load is applied to the movable jaw;

pressure sensor means operatively connected to said valve means and flow connected to said pistoncylinder means for sensing the pressure of the fluid in said piston cylinder means and opening said valve means when the pressure of said fluid exceeds the predetermined maximum;

said pressure sensor means adapted to be connected to the means for moving said movable jaw for stopping the means for moving the movable jaw when the pressure of said fluid exceeds a predetermined maximum.

2. For use in ajaw crusher apparatus for relieving excessive loads according to claim 1, said piston-cylinder means including stop means for limiting the movement of said piston means in one direction for permitting fluid in said piston-cylinder means to be pressurized to said predetermined maximum and limiting the movement of said movable jaw toward said fixed jaw when said fluid is being pressurized.

3. For use in a jaw crusher, apparatus for relieving excessive loads according to claim 1 further comprising means flow connected to said piston-cylinder means for sensing when the fluid pressure in said piston cylinder means is less than a predetermined minimum.

4. A rock crusher comprising:

a frame;

a fixed jaw mounted on said frame;

a movable jaw pivotally mounted on said frame;

a shaft rotatably mounted on said frame and adapted to be connected to means for rotating the shaft at a predetermined speed;

a Pitman operatively connected to said shaft;

a forward toggle means extending between said Pitman and said movable jaw;

piston-cylinder means mounted on said frame including cylinder means fixed to said frame and piston means movably mounted in said cylinder means;

a rear toggle means extending between said Pitman and said piston means;

said piston-cylinder means being adapted to contain a volume of hydraulic fluid at a predetermined pressure sufficient to prevent said rear toggle means from moving away from said fixed jaw and said piston means from moving into said cylinder means during normal crushing operations and insufficient to prevent the movement of said rear toggle means away from said fixed jaw and said piston means from moving into said cylinder means when an uncrushable material is encountered by said movable jaw; and

means for exhausting hydraulic fluid from said piston-cylinder means when an uncrushable material is encountered by said movable jaw;

said means for exhausting hydraulic fluid from said piston-cylinder means including valve means operatively connected to said piston-cylinder means for permitting hydraulic fluid to be exhausted from said piston-cylinder means at a flow rate at least equal to that required to permit said rear toggle to move freely away from said fixed jaw at the predetermined operating speed of said shaft when the pressure of said hydraulic fluid exceeds a predetermined maximum;

pressure sensor means operatively connected to said valve means and flow connected to said piston cylinder means for sensing the pressure of the hydraulic fluid in said piston-cylinder means and opening said valve means when the pressure of said hydraulic fluid exceeds the predetermined maximum;

said pressure sensor means being adapted to be connected to the means for rotating said shaft for stopping the means for rotating said shaft when the pressure of said hydraulic fluid exceeds the predetermined maximum.

5. A rock crusher according to claim 4 further comprising second pressure sensor means flow connected to-said piston-cylinder means and adapted to be connected to the means for rotating said shaft for stopping the means for rotating said shaft when the pressure of said hydraulic fluid is below a predetermined minimum.

6. A rock crusher comprising:

a frame;

a fixed jaw mounted on said frame;

a movable jaw pivotally mounted on said frame;

a shaft rotatably mounted on said frame and adapted to be connected to means for rotating the shaft at a predetermined speed;

a Pitman operatively connected to said shaft;

a forward toggle means extending between said Pitman and said movable jaw;

piston-cylinder means mounted on said frame including cylinder means fixed to said frame and piston means movably mounted in said cylinder means;

a rear toggle means extending between said Pitman and said piston means;

said piston-cylinder means being adapted to contain a volume of hydraulic fluid at a predetermined pressure sufficient to prevent said rear toggle means from moving away from said fixed jaw and said piston means from moving into said cylinder means during normal crushing operations and insufficient to prevent the movement of said rear toggle means away from said fixed jaw and said piston means from moving into said cylinder means when an uncrushable material is encountered by said movable jaw;

means for exhausting hydraulic fluid from said piston-cylinder means when an uncrushable material is encountered by said movable jaw;

said means for exhausting hydraulic fluid from said piston-cylinder means including valve means operatively connected to said piston-cylinder means for permitting hydraulic fluid to be exhausted from said piston-cylinder means at a flow rate at least equal to that required to permit said rear toggle to move freely away from said fixed jaw at the predetermined operating speed of said shaft when the pressure of said hydraulic fluid exceeds a predetermined maximum;

pressure sensor means operatively connected to said valve means and flow connected to said pistoncylinder means for sensing the pressure of the hydraulic fluid in said piston cylinder means and opening said valve means when the pressure of said hydraulic fluid exceeds the predetermined maximum; and

second pressure sensor means flow connected to said piston-cylinder means and adapted to be connected to the means for rotating said shaft for stopping the means for rotating said shaft when the pressure of said hydraulic fluid is below a predetermined minimum. 

1. For use in a jaw crusher including a frame, a fixed jaw mounted on said frame and a movable jaw mounted on said frame and means for moving said movable jaw alternately toward and away from said fixed jaw at a predetermined operating speed to thereby crush material between said fixed and movable jaws, apparatus for relieving excessive loads applied to said movable jaw comprising: piston-cylinder means adapted to contain a volume of fluid and including a piston means mounted in a cylinder means; said piston means operatively connected to the movable jaw of the jaw crusher and movable relative to the cylinder means and said cylinder means being fixed to said frame; means operatively connected to said piston-cylinder means for exhausting fluid in said piston-cylinder means when the fluid pressure in said piston-cylinder means exceeds a predetermined maximum whereby when an excessive load is applied to the movable jaw, it is transferred to the fluid through the piston means; said means for exhausting fluid including valve means for exhausting fluid from the piston-cylinder means at a flow rate which is at least equal to that required to permit said piston means to move freely relative to said cylinder means at the predetermined operating speed of the movable jaw when an excessive load is applied to the movable jaw; pressure sensor means operatively connected to said valve means and flow connected to said piston-cylinder means for sensing the pressure of the fluid in said piston cylinder means and opening said valve means when the pressure of said fluid exceeds the predetermined maximum; said pressure sensor means adapted to be connected to the means for moving said movable jaw for stopping the means for moving the movable jaw when the pressure of said fluid exceeds a predetermined maximum.
 2. For use in a jaw crusher apparatus for relieving excessive loads according to claim 1, said piston-cylinder means including stop means for limiting the movement of said piston means in one direction for permitting fluid in said piston-cylinder means to be pressurized to said predetermined maximum and limiting the movement of said movable jaw toward said fixed jaw when said fluid is being pressurized.
 3. For use in a jaw crusher, apparatus for relieving excessive loads according to claim 1 further comprising means flow connected to said piston-cylinder means for sensing when the fluid pressure in said piston cylinder means is less than a predetermined minimum.
 4. A rock crusher comprising: a frame; a fixed jaw mounted on said frame; a movable jaw pivotally mounted on said frame; a shaft rotatably mounted on said frame and adapted to be connected to means for rotating the shaft at a predetermined speed; a Pitman operatively connected to said shaft; a forward toggle means extending between said Pitman and said movable jaw; piston-cylinder means mounted on said frame including cylinder means fixed to said frame and piston means movably mounted in said cylinder means; a rear toggle means extending between said Pitman and said piston means; said piston-cylinder means being adapted to contain a volume of hydraulic fluid at a predetermined pressure sufficient to prevent said rear toggle means from moving away from said fixed jaw and said piston means from moviNg into said cylinder means during normal crushing operations and insufficient to prevent the movement of said rear toggle means away from said fixed jaw and said piston means from moving into said cylinder means when an uncrushable material is encountered by said movable jaw; and means for exhausting hydraulic fluid from said piston-cylinder means when an uncrushable material is encountered by said movable jaw; said means for exhausting hydraulic fluid from said piston-cylinder means including valve means operatively connected to said piston-cylinder means for permitting hydraulic fluid to be exhausted from said piston-cylinder means at a flow rate at least equal to that required to permit said rear toggle to move freely away from said fixed jaw at the predetermined operating speed of said shaft when the pressure of said hydraulic fluid exceeds a predetermined maximum; pressure sensor means operatively connected to said valve means and flow connected to said piston cylinder means for sensing the pressure of the hydraulic fluid in said piston-cylinder means and opening said valve means when the pressure of said hydraulic fluid exceeds the predetermined maximum; said pressure sensor means being adapted to be connected to the means for rotating said shaft for stopping the means for rotating said shaft when the pressure of said hydraulic fluid exceeds the predetermined maximum.
 5. A rock crusher according to claim 4 further comprising second pressure sensor means flow connected to said piston-cylinder means and adapted to be connected to the means for rotating said shaft for stopping the means for rotating said shaft when the pressure of said hydraulic fluid is below a predetermined minimum.
 6. A rock crusher comprising: a frame; a fixed jaw mounted on said frame; a movable jaw pivotally mounted on said frame; a shaft rotatably mounted on said frame and adapted to be connected to means for rotating the shaft at a predetermined speed; a Pitman operatively connected to said shaft; a forward toggle means extending between said Pitman and said movable jaw; piston-cylinder means mounted on said frame including cylinder means fixed to said frame and piston means movably mounted in said cylinder means; a rear toggle means extending between said Pitman and said piston means; said piston-cylinder means being adapted to contain a volume of hydraulic fluid at a predetermined pressure sufficient to prevent said rear toggle means from moving away from said fixed jaw and said piston means from moving into said cylinder means during normal crushing operations and insufficient to prevent the movement of said rear toggle means away from said fixed jaw and said piston means from moving into said cylinder means when an uncrushable material is encountered by said movable jaw; means for exhausting hydraulic fluid from said piston-cylinder means when an uncrushable material is encountered by said movable jaw; said means for exhausting hydraulic fluid from said piston-cylinder means including valve means operatively connected to said piston-cylinder means for permitting hydraulic fluid to be exhausted from said piston-cylinder means at a flow rate at least equal to that required to permit said rear toggle to move freely away from said fixed jaw at the predetermined operating speed of said shaft when the pressure of said hydraulic fluid exceeds a predetermined maximum; pressure sensor means operatively connected to said valve means and flow connected to said piston-cylinder means for sensing the pressure of the hydraulic fluid in said piston cylinder means and opening said valve means when the pressure of said hydraulic fluid exceeds the predetermined maximum; and second pressure sensor means flow connected to said piston-cylinder means and adapted to be connected to the means for rotating said shaft for stopping the means for rotating said shaft when the pressure of said hydraulic fluid is below a Predetermined minimum. 